During the past year methods have been explored for the release of the molybdenum cofactor from xanthine oxidase and sulfite oxidase. These studies have shown that identical substances, judged by chemical and physical properties can be released from their non-covalent interactions with both enzymes. The extreme instability of the cofactor has made it difficult to isolate a homogeneous molecule for structural studies. For the current year, attempts will be directed towards stabilizing the cofactor or otherwise minimizing modifications, to facilitate isolation of a single form of the cofactor for structural studies. The possibility of in vivo labeling of the cofactor will be explored. Knowledge regarding cofactor structure could conceivably be derived from biosynthetic studies. For this suitable mutants of E. coli, N. crassa or Drosophila will be used. Little is known regarding the biological function of the molybdoenzyme aldehyde oxidase. Structural and enzymological studies on this enzyme will be initiated with the ultimate view of understanding the biological function of the enzyme.